The present invention relates to an automatic laundry washer operable to perform laundering phases and centrifuging by means of a rotatable drum containing the laundry. In Italian Patent No. 1187301, filed on Nov. 27, 1985 in the name of the present applicant, there is described a laundry washer operable to recirculate the lye collected in a receptacle at the bottom of the laundering tub, thereby returning the lye to the rotating drum so as to soak the laundry contained therein. In the course of the execution of the laundering cycle, the drum is adapted to be rotated at the centrifuging speed for short periodic intervals, in order to extract from the laundry a sufficient amount of the lye as required for the operation of the lye recirculation pump.
During these periodic centrifuging intervals, the laundering tub contains not only water, but also detergent, by contrast to what happens during the conventional final spin-drying phase.
When these periodic centrifuging intervals exceed a certain duration of, for instance, three seconds, the centrifuging operation may cause the formation of an excessive amount of foam in the laundering tub, which may then escape from the machine, for instance, by way of the detergent distributor.
The invention described in the above cited patent conveys the teaching of eliminating the disadvantageous formation of an excessive amount of foam by the use of a novel level sensor or pressure switch and an associated circuit for deenergizing the electric motor rotating the drum to thereby interrupt the centrifuging period in response to lye and foam rising above a predetermined level in the course of the centrifuging period. As a result, the high-speed rotation of the drum occurring for determined periods in the course of a laundering phase comes to a halt as soon as the pressure within the laundering tub attains a determined limit value in response to a combination of parameters contributing to the formation of foam, which include, in particular, the dynamic pressure acting on the pressure switch as a result of the increased centrifuging speed of the rotating drum.
This laundry washer has been found to be particularly advantageous, since it permits a considerable savings of water, detergents and electric energy to be achieved.
This laundry washer nevertheless still has a shortcoming which is noted not during the periodic centrifuging phases for the alimentation of the recirculating pump, but rather during the centrifuging phases and simultaneous discharge phases for discharging the liquid from the tub. When the novel pressure switch acts to stop the rotation of the drum in the course of one of these phases, the program control unit continues to run for controlling the selected cycle in the normal manner, so that the discharge pump is stopped after a certain time.
During this normally very short interval there is not enough time for the pump to completely discharge all of the lye and foam remaining in the tub, so that these substances are then dissolved in the water of the subsequently charged rinsing bath.
The disadvantageous effects of foam and lye remaining in the laundering tub also come about even in the case of normal operation of the discharge pump during the discharge phase, when there are external obstacles to the normal liquid flow due, for instance, to an excessive length and/or installation height of the discharge hose, to a strong throttling effect thereof, to the discharge filter being obstructed, or to the fact that the mains voltage is insufficient for properly energizing the discharge pump, so that it is incapable of completely draining the laundering tub.